Conventional electrolytic tin plating processes for many years have used phenol sulfonic acid (PSA) as the major acidifying constituent of the electrolyte. Being organic and based on phenol, this compound has come under criticism in recent years because of the waste disposal problems it creates. The organic sludge it generates, to a great extent in combination with tin compounds, is complex and contains a high concentration of phenolic compounds generally considered to be undesirable.
In addition, PSA is difficult to analyze in the electrolytic plating line, is toxic, requires a large volume of solution for the plating process, and consumes excessive amounts of power because of its relatively low conductivity.
An acceptable substitute for PSA which eliminates or minimizes the above recited shortcomings would be beneficial to the industry both economically and environmentally.
Electrolytic tin plating requires a source of tin ion and a conductive bath to promote the process of tin deposition. The conductivity of an acid electrolyte for tin plating generally improves with its acidity; tin is conventionally introduced in the form of tin sulfate.
An electrolyte comprising tin sulfate and sulfuric acid in the range of 100-140 g/l has been used for depositing tin on copper wires, foils and other electronic parts to make them solderable, and is described in Metal Finishing guide Book and Directory Issue, v. 94, No. 1A, 1996 p 224-297; see Table VIII.
A non-continuous process for depositing bright tin on unspecified substrates apparently for use in the electronics industry is described by Commander and Paneccasio in U.S. Pat. No. 5,061,351 wherein the novel pyridinyl brightening agents are said to be useful through wide ranges of current densities, sulfuric acid concentrations, and metal ion concentrations.
In U.S. Pat. No. 3,860,502, Johnson introduced the use of ethoxylated naphthol sulfonic acid (ENSA) as a brightening agent providing reduced foam and sludge in plating solutions containing 6-30 g/l of free acid (calculated as H.sub.2 SO.sub.4), which may be added as sulfuric acid--see column 4, lines 27-29. See also JP 6346273, which also uses ENSA and a sludge suppressor such as hydroquinone or resorcinol in a continuous electrolytic tin process, with H.sub.2 SO.sub.4 concentrations of 5-50 g/l. The authors observe that higher concentrations of sulfuric acid will lead to less sludge formation, but that high sulfuric acid concentrations will cause evolution of hydrogen gas and low tin deposition efficiency due to redissolving of the tin after it is deposited. Accordingly, much of the disclosure of JP 6346273 is devoted to the efficacy of various sludge suppressants used in conjunction with lower concentrations of sulfuric acid, i.e. in the range of 5-50 g/l.
In Japanese Patent Application (Kokai) Hei JP 6-346272, Itatsu and Oyagi disclose a high current density tin plating process which uses a bath containing, as a major component, 5-50 g/L of sulfuric acid, 40-100 g/L of tin (II), brightening agent, and sludge suppressing agent. The deposition is conducted at a current density of 50 A/dm.sup.2 "or higher." The authors say that if the concentration of sulfuric acid is higher than 50 g/L, "the dissolution of the steel strip will become serious, so that the iron concentration in the bath will increase."
The presence of iron ions in the bath can be a significant problem, because, in addition to the corrosion of the strip, their generation implies a dissipation of current density to bring about the corrosion of the strip, thus making the process less efficient. High current density has a clear positive correlation to process efficiency. The presence of Fe.sup.+3 ions in the bath also tends to contribute to the generation of sludge by promoting the oxidation of stannous ions to stannic ions, which form the insoluble hydroxide; the Fe.sup.++ ions formed in the reaction can easily be oxidized again to Fe.sup.+3, which again is available for the undesired conversion of the stannous ions to stannic.
Ethoxylated nonylphenols, and "Tergitol NP9" in particular, have been proposed for use in various contexts in tin plating. See Passel's U.S. Pat. No. 3,755,096, for example, which makes brightly tinned objects in an apparently static process wherein the bath essentially includes acrylic acid and a further custom-made "brightener". See also U.S. Pat. No. 2,457,152 by Hoffman, who screened a large number of polyethers for use in tin plating processes--see his general description of the phenolics at col. 10, lines 43-60. However, these workers were not faced with the acute problem of minimizing sludge formation while continuing to produce high quality product.